Method of fabricating crystalline island on substrate

ABSTRACT

Certain electronic applications, such as OLED display back panels, require small islands of high-quality semiconductor material distributed over a large area. This area can exceed the areas of crystalline semiconductor wafers that can be fabricated using the traditional boule-based techniques. This specification provides a method of fabricating a crystalline island of an island material, the method comprising depositing particles of the island material abutting a substrate, heating the substrate and the particles of the island material to melt and fuse the particles to form a molten globule, and cooling the substrate and the molten globule to crystallize the molten globule, thereby securing the crystalline island of the island material to the substrate. The method can also be used to fabricate arrays of crystalline islands, distributed over a large area, potentially exceeding the areas of crystalline semiconductor wafers that can be fabricated using boule-based techniques.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional PatentApplication No. 62/007,624, filed on Jun. 4, 2014, which is incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to a method of fabricating one or morecrystalline islands of an island material abutting a substrate.

BACKGROUND OF THE INVENTION

Certain electronic applications, such as OLED display back panels,require small islands of high-quality semiconductor material distributedover a large area. This area can be 50 inches diagonally and larger, andexceeds the sizes of crystalline semiconductor wafers that can befabricated using the traditional boule-based techniques.

WO 2013/053052 A1, incorporated herein by reference, disclosesfabricating a large number of small, loose, crystalline semiconductorspheres. The spheres are then distributed on a patterned substrate andaffixed to the substrate at predetermined locations to form an array ofspheres on the substrate. Planarizing the spheres exposes across-section of each sphere, thereby providing an array ofhigh-quality, crystalline semiconductor islands for device fabricationon a globally planarized surface.

U.S. Pat. No. 4,637,855, incorporated herein by reference, disclosesfabricating spheres of silicon on a substrate by applying a slurry ofmetallurgical grade silicon to the substrate and then patterning theslurry layer to provide regions of metallurgical silicon of uniformsize. The substrate is then heated to melt the silicon, which then beadsto the surface to form molten spheres of silicon, which are then cooledto crystallize them. The spheres have very weak adhesion to thesubstrate and are easily detached from the substrate by simply knockingthem loose. The loose spheres are collected and undergo furtherprocessing.

US 2012/0067273 A1, incorporated herein by reference, disclosesfabricating silicon wafers by bringing a substrate into contact with areservoir of molten silicon, forming a layer of solid silicon on thesubstrate, and subsequently detaching the solid layer from thesubstrate. The disclosed method can potentially be used to fabricatesilicon wafers with large areas.

SUMMARY OF THE INVENTION

Provided herein is a method of fabricating one or more crystallineislands of an island material abutting a substrate. For each crystallineisland, particles of the island material are deposited abutting thesubstrate, and the substrate and the particles are then heated to meltand fuse the particles to form a respective molten globule. Thesubstrate and the respective molten globules are then cooled tocrystallize the molten globules, thereby securing the crystallineislands to the substrate.

This method can allow for fabricating crystalline islands usingparticulate starting materials. In addition, in some embodiments thecrystalline islands being secured to the substrate can allow for furtherprocessing of the islands; for example, planarizing at least a portionof each crystalline island to expose a cross-section of each island. Ifthe crystalline islands are sufficiently high-quality crystallinesemiconductors, these cross-sections can then be used to fabricateelectronic devices. This method can also allow for fabricating arrays ofcrystalline islands, distributed over an area potentially exceeding theareas of crystalline semiconductor wafers that can be fabricated usingtraditional boule-based techniques.

According to an aspect of the present invention, there is provided amethod of fabricating a crystalline island of an island material, themethod comprising depositing particles of the island material abutting asubstrate, heating the substrate and the particles of the islandmaterial to melt and fuse the particles to form a molten globule, andcooling the substrate and the molten globule to crystallize the moltenglobule, thereby securing the crystalline island of the island materialto the substrate.

The crystalline island can comprise a single crystal of the islandmaterial or a polycrystalline form of the island material.

The method can further comprise planarizing at least a portion of thecrystalline island to expose a cross-section of the crystalline island.

The securing can comprise the molten globule wetting the substrate at awetting angle smaller than about 90 degrees and the crystalline islandadhering to the substrate.

The depositing can comprise defining a depression in the substrate, andtransferring the particles of the island material into the depression.

When the depositing comprises defining a depression and transferring theparticles of the island material into the depression, the securing cancomprise a portion of a surface of the depression enveloping a portionof a surface of the crystalline island.

When the depositing comprises defining a depression and transferring theparticles of the island material into the depression, the depression canbe shaped to have at least one vertex.

When the depositing comprises defining a depression and transferring theparticles of the island material into the depression, the depression cancomprise a first depression, and a second depression within the firstdepression, the second depression smaller and deeper than the firstdepression.

When the depositing comprises defining a depression and transferring theparticles of the island material into the depression, the transferringcan comprise one or more of doctor-blading the particles of the islandmaterial into the depression, and electrostatic deposition of theparticles of the island material into the depression using a chargedpin.

When the depositing comprises defining a depression and transferring theparticles of the island material into the depression, the transferringcan comprise flowing a suspension onto the substrate and into thedepression, the suspension comprising a dispersion of the particles ofthe island material in a carrier medium, and squeegeeing the suspensionlocated on the substrate outside the depression; and the heating canfurther comprise eliminating the carrier medium prior to the melting andfusing the particles of the island material.

The cooling can comprise one or more of oxidizing an outer surface ofthe molten globule, super-cooling the molten globule, and applying aphysical impact to the substrate.

The depositing can comprise transferring particles of the islandmaterial into a through hole in the substrate.

When the depositing comprises transferring particles of the islandmaterial into a through hole in the substrate, pressure can be appliedat a second end of the through hole to push the molten globule partiallyout of a first end of the through hole to form a convex meniscus.

When the depositing comprises transferring particles of the islandmaterial into a through hole in the substrate, the securing can comprisea portion of a surface of the through hole enveloping a portion of asurface of the crystalline island.

When the depositing comprises transferring particles of the islandmaterial into a through hole in the substrate, the method can furthercomprise after the cooling to crystallize the molten globule,planarizing a portion of the meniscus to expose a cross-section of thecrystalline island.

When the depositing comprises transferring particles of the islandmaterial into a through hole in the substrate, a portion of a surface ofthe substrate outside the through hole and adjacent a first end of thethrough hole can have a wetting angle with the molten globule of lessthan about 90 degrees.

The depositing can comprise dispersing the particles of the islandmaterial in a carrier medium to create a suspension and transferring thesuspension onto the substrate, and the heating can further compriseeliminating the carrier medium prior to the melting and fusing theparticles of the island material.

When the depositing can comprise dispersing the particles of the islandmaterial in a carrier medium to create a suspension and transferring thesuspension onto the substrate, and the heating can further compriseeliminating the carrier medium prior to the melting and fusing theparticles of the island material, the transferring can comprise one ormore of stamping the suspension onto the substrate, screen printing thesuspension onto the substrate, inkjet printing the suspension onto thesubstrate, and spin-coating the suspension onto the substrate, andlithographically patterning the spin-coated suspension.

The depositing can comprise dispersing the particles of the islandmaterial in a carrier medium to create a suspension, forming thesuspension into a sheet, causing the sheet to solidify to form a solidsheet, patterning the solid sheet by removing one or more portions ofthe sheet to form a patterned sheet, and overlaying the patterned sheeton the substrate; and the heating can further comprise eliminating thecarrier medium prior to the melting and fusing the particles of theisland material.

The molten globule can have a first wetting angle with a first portionof a surface of the substrate in contact with the molten globule, and asecond wetting angle with a second portion of the surface of thesubstrate, the second portion abutting the first portion, and the secondwetting angle being greater than the first wetting angle.

An area of the substrate in contact with the molten globule can compriseone or more of one or more guiding protrusions, one or more guidingdepressions, and a metallic grid for controlling initiation ofcrystallization as the molten globule is cooled.

The coefficient of thermal expansion (CTE) of the substrate at atemperature within about 20° C. of the melting point of the islandmaterial can match the CTE of the island material at the melting pointof the island material.

The island material can comprise silicon.

The substrate can comprise alumina.

The securing can comprise over-coating the crystalline island and thesubstrate with an over-coating layer to form a stack whereby thecrystalline island is sandwiched between the substrate and theover-coating layer.

When the securing comprises over-coating the crystalline island and thesubstrate with an over-coating layer to form a stack whereby thecrystalline island is sandwiched between the substrate and theover-coating layer, the method can further comprise planarizing thestack to expose a cross-section of the crystalline island.

According to a further aspect of the present invention, there isprovided a method of fabricating a crystalline island of an islandmaterial, the method comprising depositing particles of the island'material on a first substrate, sandwiching the particles of the islandmaterial between the first substrate and a second substrate by placingthe second substrate adjacent the first substrate, heating the firstsubstrate, the second substrate, and the particles of the islandmaterial to melt and fuse the particles to form a molten globule,cooling the first substrate, the second substrate, and the moltenglobule to crystallize the molten globule, thereby forming thecrystalline island of the island material.

The cooling can further comprise one or more of applying a pressurepulse to the molten globule, adding a seed crystal to the moltenglobule, and super-cooling the molten globule.

The first substrate can have a first area being a portion of a surfaceof the first substrate that comes into contact with the molten globule,and the second substrate can have a second area being a portion of asurface of the second substrate that comes into contact with the moltenglobule, and one or more of the first area and the second area cancomprise one or more of one or more protrusions, one or moredepressions, a metallic grid, for controlling initiation ofcrystallization as the molten globule is cooled.

DESCRIPTION OF THE DRAWINGS

For a better understanding of the various implementations describedherein and to show more clearly how they may be carried into effect,reference will now be made, by way of example only, to the accompanyingdrawings in which:

FIG. 1 depicts a method of fabricating a crystalline island abutting asubstrate, according to non-limiting implementations.

FIG. 2 depicts a top perspective view of crystalline islands on asubstrate, according to non-limiting implementations.

FIG. 3 depicts a top perspective view of a substrate having adepression, according to non-limiting implementations.

FIG. 4 depicts a collection of possible shapes for a depression,according to non-limiting implementations.

FIG. 5 depicts a top perspective view of a substrate having a depressionwithin a depression, according to non-limiting implementations.

FIGS. 6a-d depict a cross-section of a substrate having a depressionwithin a depression at different stages of forming a crystalline islandin the depressions, according to non-limiting implementations.

FIG. 7 depicts a cross-section of a substrate having a depression withina depression, according to non-limiting implementations.

FIG. 8 depicts a cross-section of a substrate having a depression withina depression, according to non-limiting implementations.

FIGS. 9a-c depict a cross-section of a substrate having a through hole,at different stages of forming a crystalline island in the through hole,according to non-limiting implementations.

FIG. 10 depicts a method of depositing particles of an island materialon a substrate, according to non-limiting implementations.

FIG. 11 depicts a cross-section of a crystalline island on a substrate,both over-coated, according to non-limiting implementations.

FIG. 12 depicts a cross-section of a crystalline island on a substrate,both over-coated, according to non-limiting implementations.

FIG. 13 depicts a method of fabricating a crystalline island sandwichedbetween two substrates, according to non-limiting implementations.

FIG. 14 depicts a cross-section of a molten globule sandwiched betweentwo substrates, according to non-limiting implementations.

FIG. 15 depicts a top plan view of an array of pyramids and abuttingcrystal grains on a substrate, according to non-limitingimplementations.

FIG. 16 depicts a cross-section of a molten globule sandwiched betweentwo substrates, according to non-limiting implementations.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention is reflected in method 100 shownin FIG. 1. Method 100 can be used to fabricate a crystalline islandabutting a substrate, on top of or inside a substrate. First, as shownin box 105, island material can be deposited abutting the substrate, onor in the substrate. The island material can be in particulate form.Particles of the island material can be deposited at a predeterminedlocation relative to the substrate, which can include, but is notlimited to, deposition on all or a portion of the surface of thesubstrate. Particles of the island material can be deposited as asubstantially pure powder, a powder with some additives or impurities,or as particles of the island material suspended in a carrier medium.Additives can be used to dope, alloy, or otherwise compound the islandmaterial.

Subsequently, as shown in box 110, the substrate and the particles ofthe island material can be heated. The heating can be by conduction,convection, and/or radiative heating, and can be performed in a furnace,kiln, or other suitable heating apparatus known to the skilled person.The heating melts and fuses the particles of the island material to forma molten globule of the island material. When a carrier medium is usedto deposit the island material, the heating can evaporate, burn off, orotherwise eliminate the carrier medium before the melting and fusing ofthe island material particles. The island and substrate materials can bechosen so that the substrate does not melt at the temperature requiredto melt and fuse the particles of the island material.

Subsequently, as shown in box 115, the substrate and the molten globuleare cooled to crystallize the molten globule, thereby securing theresulting crystalline island to the substrate. In some embodiments, theislands are secured strongly enough to allow mechanical polishing, orother abrasive processing, of the island to expose a cross-section ofthe island, without dislodging the island from the substrate.

FIG. 2 shows an array of crystalline islands 210 formed on substrate205. Method 100 can be used to fabricate one or any number ofcrystalline islands. When a plurality of islands is formed, they can bearranged in an ordered array, or can be distributed on the substratewithout perceptible periodic ordering. In some embodiments, the positionof each island 210 relative to substrate 205 is known to allow forsubsequent processing of the islands.

Islands 210 can be single-crystalline or poly-crystalline.Nano-crystalline and amorphous islands can also be formed. In someembodiments after islands 210 are formed, they are planarized, abraded,or otherwise operated upon so that some material is removed from thesurfaces thereof to expose a cross-section of each island. This exposedcross-section can then be used to fabricate electronic devices, forexample when the islands are formed from a semiconductor such assilicon.

When the starting particulate island material has impurities, theprocess of melting and crystallizing set out in method 100 can reducethe impurities inside the crystalline islands by pushing the impuritiestowards the surface of each molten globule as a crystalline latticebegins to form inside the molten globule, a process known as gettering.The lattice then tends to exclude any impurities that would interferewith its ordered arrangement of atoms, thereby excluding at least aportion of the impurities from the inside of the crystal. When thecrystalline islands are poly-crystalline, the impurities are pushed tothe grain boundaries between the crystals.

In some embodiments, particles of the island material are deposited ontothe substrate by transferring the island material into one or moredepressions defined in the substrate surface. FIG. 3 shows depression310 in substrate 305. In some embodiments a plurality of depressions canbe defined, and they can be arranged into an ordered array. Thedepressions can be defined lithographically, or using other means knownto the skilled person, including but not limited to laser ablation orlocalized etching using a photolithographically defined mask.

The depression 310 contains the molten globule, and can serve to moreprecisely locate the molten globule, and the resulting crystallineisland, relative to the substrate 305. In addition, the shape of thedepression can guide the crystallization process by providing nucleationsites for initiating crystallization. In some embodiments the depressioncan have one or more vertexes 425 as shown in FIG. 4 in relation todepressions 405, 410, 415, and 420. The vertex 425 can be at the end ofa taper such as in the case of depression 420, or part of a reversetaper, as shown in depressions 405 and 415. A depression can havemultiple vertexes as shown in depressions 410 and 415. Having multiplenucleation sites in the form of vertexes 425 increases the likelihoodfor the island being poly-crystalline and decreases the likelihood offormation of a single-crystalline island. Nucleation sites can also bethree-dimensional smaller depressions into or protrusions from thesurface of depression 310. Such smaller depressions can have one or morevertexes (not shown). These vertexes can in turn serve as a nucleationsite for crystallization of the molten globule.

As shown in FIG. 5, in some embodiments, substrate 505 can have a largerand shallower depression 510, within which there can be a smaller anddeeper depression 515.

FIGS. 6a -d show a cross-section of a substrate 605, with a largerdepression 610 and a smaller and deeper depression 615 within depression610. As shown in FIG. 6b , particles of the island material 620 can bedeposited to fill both the larger and the smaller depressions 610 and615. As shown in FIG. 6c , when particles 620 are melted, they fuse toform molten globule 625. The surface tension of the molten globule canpull it into an approximately spherical shape, which is then cooled andcrystallizes to form a crystalline island. As shown in FIG. 6d , thecrystalline island and/or the substrate 605 can be planarized to yield acrystalline island 630 in substrate 605. Depression 610 serves as areservoir for particles of the island material which eventually form themolten globule 625. Depression 615, in turn, serves to locate the moltenglobule 625 and initiate its crystallization using vertexes discussedabove in relation to FIG. 5.

The relative volume of depressions 610 and 615 can determine the size ofthe molten globule. This, combined with the depth of depressions 610 and615 can determine the size of cross-section of crystalline island 630available at different depths of substrate 605. When particles of islandmaterial 620 fuse to form molten globule 625 leaving at least a portionof the volume of depression 610 empty, this empty space can beback-filled after molten globule 625 crystallizes. This back-filling canhelp to create a planar surface.

FIG. 7 shows a side elevation cross-section of a substrate 705 havinglarge depression 710, and small depression 715 within large depression710. As shown in the cross-section, the surface of small depression 715envelops more than half of molten globule 725 (shown in dotted line)thereby physically securing the crystallized molten globule 725 to thesubstrate. Even in embodiments where the surface of small depression 715envelops less than half of the molten globule 725, the enveloping canstill contribute to physically securing the crystalline island to thesubstrate. The shape of depression 715 is not limited to a portion of asphere. Any suitable shape can be used. When it is desirable tophysically secure the crystalline island to the substrate, the shape ofdepression 715 can be used whereby the molten globule can flow into theshape, but the solid crystalline island cannot be physically removedfrom depression 715. An example of such shapes for depression 715 can beany shape where the opening of depression 715 is smaller than thelargest dimension of the crystalline island that must pass through theopening in order for the crystalline island to be removed fromdepression 715.

FIG. 8 shows a side elevation cross-section of a substrate 805 having alarge depression 810 and a small depression 815 within large depression810. Small depression 815 has protrusion 820 extending into the space tobe occupied by molten globule 840. Protrusion 820 can have at least onevertex 825. Instead of, or in addition to, protrusion 820, smalldepression 815 can have a further depression 830 in the surface of smalldepression 815. Further depression 830 may have at least one vertex 835.Further depression 830 can also be described as a protrusion of thespace to be occupied by molten globule 840 into small depression 815.Depression 815 may have any number or combination of protrusions 820 andfurther depressions 830. Vertexes 825 and 835 can form a nucleation sitefor initiating the crystallization of molten globule 840. Once moltenglobule 840 is crystallized, protrusion 820 and further depression 830can contribute to physically securing the crystallized island intodepression 815, and in turn securing the crystalline island to substrate805.

The securing means discussed in relation to FIGS. 7 and 8 can also beused in a single-stage depression such as depression 310 shown in FIG.3. In addition to these means of physically securing the crystallineisland to the substrate, surface adhesion can also be used to secure thecrystalline island to the substrate. For example, if the molten globulehas a wetting angle with the substrate of less than about 90°, themolten globule sufficiently wets the substrate surface and contributesto the adhesion of the crystallized island to the substrate surface.

When the particulate island material is in the form of a loose powder,it can be transferred into the depression in the substrate using meansincluding but not limited to: 1) doctor-blading the powder into thedepression; and 2) electrostatically depositing the powder into thedepression using charged pins to pick and then deposit the powder intothe depression.

When the particulate island material is in the form of a suspension ofthe particles in a carrier medium, the suspension can be flowed onto thesubstrate to fill the depression and then squeegeeing the excesssuspension located outside the depression from the surface of thesubstrate. When such a carrier medium is used, during the heating stepit can be evaporated, burnt off, or otherwise eliminated before themelting and fusing of the particulate island material.

In the cooling stage, cooling alone can be sufficient to initiate thecrystallization of the molten globule. Other techniques can be used tofacilitate or more finely control the initiation and progress of thecrystallization. For example, the molten globule can be super-cooledbelow its melting point. Super-cooling can take the form of cooling themolten globule to less than around 300° C. below its melting pointbefore the crystallization starts. Applying a physical impact or shockto the substrate bearing the molten globule can also set offcrystallization. This can also be used when the molten globule issuper-cooled. In addition, the surface of the molten globule can beexposed to different chemical reactants, such as oxygen, to furtherguide the crystallization process. The oxygen can form a thin layer or“skin” on the surface of the molten globule which serves to isolate themolten silicon from the substrate and can serve to increase the surfacetension of the globule.

FIG. 9a shows an embodiment where substrate 905 has a through hole 910filled with particulate island material 915. Hole 910 has a first end940 and a second end 945. Hole 910 can be filled with particulate islandmaterial 915 using doctor-blading, Hole 910 can also be filled with aliquid suspension comprising the particulate island material dispersedin a carrier medium. The suspension can be flowed onto the substrate andinto hole 910, and then the excess suspension located outside hole 910can be squeegeed off the surface of the substrate. After filling hole910, substrate 905 and island material 915 can be heated to melt andfuse the island material into molten globule 935. When such a carriermedium is used, during the heating step it can be evaporated, burnt off,or otherwise eliminated before the melting and fusing of the particulateisland material. Optionally, substrate 905 can be flipped before theheating. For example, the flipping can be used when hole 910 has aclosed end, to point the open end of hole 910 towards the earth'sgravitational force.

FIG. 9b shows molten globule 935 forming a convex meniscus 920 extendingout of the first end 940 of hole 910. Meniscus 920 can form under theforce of gravity. In addition, if surface 925 of substrate 905 adjacentfirst end 940 of hole 910 has a low wetting angle with the moltenglobule 935, this can encourage the molten globule 935 to wet thesurface 925 of substrate 905 and for the meniscus 920 to form and extendout of first end 940 of hole 910. Furthermore, molten globule 930 can beencouraged to extend out of hole 910 and form meniscus 920 if pressureis applied against the molten globule 920 through the second end 945 ofhole 910 to push molten globule 920 out of first end 940 of hole 910. Asshown in FIG. 9c , once molten globule 935 crystallizes, meniscus 920can be polished and/or planarized to expose a cross-section ofcrystalline island 950 and to form a crystalline island 950 in substrate905.

FIGS. 9a-c show one hole 910, but a plurality of holes can be used. Theholes can be arranged in an ordered array. Crystalline islands can besecured to substrate 905 by respective holes, such as hole 910,enveloping and physically securing the respective island 950 and/or thesurface adhesion of the crystalline island 950 to the surface of hole910. Adhesion can be stronger when the wetting angle between the moltenglobule 935 and the surface of hole 910 is smaller than about 90°.

In another implementation (not shown), particles of island material canbe dispersed in a carrier medium to form a suspension. The suspensioncan then be transferred onto the substrate. Next, the substrate and thesuspension can be heated, which can evaporate, burn off, or otherwiseeliminate the carrier medium. The heating can also melt and fuse theparticles of the island material to form a molten globule. The coolingand securing can be carried on as previously described. A wetting angleof less than about 90° between the molten globule and the substrate cancontribute to stronger adhesion between the substrate and crystallineisland and to securing the crystalline island to the substrate.

The suspension can be transferred to the substrate using techniquesincluding, but not limited to, one or more of stamping, screen printing,or inkjet printing of the suspension onto the substrate followingprocedures known in the art. The suspension can also be spin-coated toform a layer on the substrate. This layer can then be lithographicallypatterned to define one or more regions on the substrate where particlesof the island material are present, and other regions where islandmaterial particles are absent.

There may not be any depressions in this embodiment. However, thesubstrate surface can be patterned to have areas of higher wetting angleand other areas of lower wetting angle with the molten globule. Themolten globule will tend to form on the areas of lower wetting angle.The patterning of areas with low wetting angle can serve as a means offurther locating the molten globule, and thus the crystalline island onthe substrate. This can be applied to one crystalline island or aplurality of crystalline islands. Methods for patterning a substrate tohave high and low wetting angle areas are well known in the art, and caninclude applying a patterned mask to the surface followed by subjectingthe unmasked areas to chemical modification or deposition of othermaterials, such as SiO₂, on the unmasked areas.

FIG. 10 shows a further embodiment of the present invention reflected inmethod 1000 for depositing particles of the island material on thesubstrate. First, as shown in box 1005, the particles can be dispersedin a carrier medium to create a suspension. Next, as shown in box 1010,the suspension can be formed into a sheet by spreading or spin coatingthe suspension or using other methods known in the art. Next, as shownin box 1015, the sheet of the suspension can be transformed into a solidsheet. This can be accomplished by drying, baking, cross-linking, orotherwise solidifying the suspension. Next, as shown in box 1020, thesolid sheet can be patterned by cutting away or removing one or moreportions of the sheet to form a patterned sheet. The pattern can beapplied using a mechanical punch, lithographically, or using other meansknown in the art. Next, as shown in box 1025, the patterned sheet can beoverlaid on the substrate.

At this stage the remaining steps of heating and cooling-and-securingcan be applied as previously described. During heating, the carriermedium can be evaporated, burnt off, or otherwise eliminated beforemelting and fusing of particles of the island material. A wetting angleof less than 90° between the molten globule and the substrate cancontribute to adhesion of the crystalline island to the substrate. Thisembodiment can be used to make a single island or a plurality ofcrystalline islands on the substrate, which can be arranged in anordered array.

In the embodiments where there is no depression, initiation of thecrystallization of the molten globule can still be guided andcontrolled. One or more guiding depressions into and/or guidingprotrusion from the substrate surface coming into contact with themolten globule can initiate crystallization. These guiding depressionsand protrusions can have at least one vertex to provide an initiationpoint for the crystallization process.

Alternatively, the shape of the contact area of the molten globule withthe substrate can be controlled to provide an initiation point forcrystallization. By patterning the relatively low and high wetting angleareas on the substrate, the molten globule can be made to wet or contactthe substrate along a patterned lower wetting angle shape while avoidingthe higher wetting angle areas of the substrate. The shape of the lowwetting angle area can be any of the shapes discussed above in relationto FIG. 4. The shape can have at least one vertex to provide aninitiation point for the crystallization.

Another means of controlling initiation of crystallization can bedepositing a metallic grid on the substrate, at least over the areas ofthe substrate that come into contact with the molten globule. Thedeposited metal can act as an initiation point for the crystallizationof the molten globule. The grid can be made of other materials, such asrefractories or Ni. The deposited material can have other shapes such asdots or other patterns of deposited material that may not constitute agrid.

In other embodiments, after the crystalline islands form, the islandsand the substrate can be over-coated. FIG. 11 shows a cross-section ofsubstrate 1105 and crystalline island 1110 over-coated with layer 1115.This assembly forms a stack where the crystalline island 1110 issandwiched between substrate 1105 and cover-coating layer 1115. Thestack can then be planarized to remove some of the material comprisingthe stack and expose a cross-section of the crystalline island.

The over-coating layer 1115 can be deposited using any suitable physicalor chemical deposition method including but not limited to spin coatingor electrostatically-applied powder coating. The layer can be a thinlayer, such as layer 1115 in FIG. 11, or can be a thicker layer such aslayer 1215 shown in cross-section in FIG. 12. When an over-coating layeris used, it can physically secure the crystalline island to thesubstrate by sandwiching the crystalline island between the substrateand the over-coating layer, as shown in FIGS. 11 and 12. This securingaction of the over-coating layer can contribute to securing to thesubstrate crystalline islands having a large wetting angle, andtherefore small contact area, with the substrate. Large wetting anglescan, for example, be angles greater than about 90°. Over-coating can beapplied to a plurality of islands on a substrate.

Another implementation of the present invention is reflected in method1300 shown in FIG. 13. Box 1305 shows depositing particles of an islandmaterial on a first substrate. Particles can be deposited as loosepowder or in a suspension as described above. The deposition can bepatterned and/or at specified locations relative to the substrate. Next,as shown in box 1310, the particles of the island material can besandwiched between the first substrate and a second substrate placedadjacent the first substrate. Next, as shown in box 1315, the substrateand the particles can be heated to melt and fuse the particles into amolten globule, without melting the first substrate or the secondsubstrate.

When the particles are deposited as a suspension in a carrier medium,the heating step can evaporate, burn off, or otherwise eliminate thecarrier medium before melting and fusing the particles. Next, as shownin box 1320, the substrates and the molten globule can be cooled tocrystallize the molten globule, thereby forming a crystalline island.

The sandwiching, described in box 1310, and the heating described in box1315 can be performed in the opposite order, i.e. the molten globule canform before it is sandwiched between the first and the secondsubstrates. FIG. 14 shows molten globule 1415 sandwiched between firstsubstrate 1405 and second substrate 1410.

When the wetting angle between molten globule 1410 and both firstsubstrate 1405 and second substrate 1410 is large, the crystallizedisland can adhere more weakly to the substrate. The weak adhesion canfacilitate removing the crystallized island from the substrate to form afree-standing wafer.

Features on the surfaces of one or both of the first and secondsubstrates can be used to initiate crystallization of the moltenglobule. FIG. 15 shows an array of pyramids 1505 that can be formed onthe region of the substrate that comes into contact with the moltenglobule. Pyramids 1505 can protrude from the substrate surface or formdepressions into the substrate surface. Instead of a pyramid, othershapes can be used, such as a cone. These shapes can have a vertex, asdoes pyramid 1505, to act as an initiation site for the crystallizationof the molten globule.

Each pyramid 1505 initiates crystallization to form grain 1510, whicheventually abuts upon neighboring grains at grain boundaries 1515. Usingthis method, a molten globule can be patterned into a poly-crystallineform. As discussed above, during crystallization at least some of theimpurities in the molten globule can be pushed towards the grainboundary regions. This process can leave the central region of grain1510 with relatively fewer impurities yielding a higher quality crystalfor post-processing, such as device fabrication on grain 1510. Thisprocess can isolate the grain boundaries to regions where devices willnot be fabricated in subsequent processing. Although FIG. 15 shows fourgrains 1510 of uniform size, grains 1510 can also be of different sizes.

FIG. 16 shows a cross-section of molten globule 1615 sandwiched betweentwo substrates 1605 and 1610. As discussed above, one or both of thesubstrates can include depressions or protrusions to guide thecrystallization process. Substrate 1605 can have protrusions 1620 incontact with molten globule 1615. Substrate 1610, in turn, can havedepressions 1625 in contact with molten globule 1615. The protrusions1620 and depressions 1625 can be of different shapes and any numbers ofthem can be used in/on one or both of first substrate 1605 and secondsubstrate 1610. In addition or instead of protrusions and depressions, agrid or array of a metal or other material deposited on one or both ofthe first and the second substrate where those substrates come intocontact with the molten globule can also be used to initiate and guidethe crystallization process.

The cooling as shown in box 1320 of FIG. 13 can also includesuper-cooling the molten globule. The super-cooling can include coolingthe molten globule to a temperature below about 300° C. below itsmelting point before crystallization begins. A pressure pulse ormechanical impact can also be applied to the molten globule in asuper-cooled state or otherwise. A seed crystal can also be added to themolten globule in a super-cooled state or otherwise.

In some embodiments, the coefficient of thermal expansion (CTE) of thesubstrate at a temperature within about 20° C. of the melting point ofthe island material can be matched to the CTE of the island material atthe melting point of the island material. This matching of CTE canreduce stresses between the island material and the substrate as eachone cools and contracts. Lower stresses can facilitate making of higherquality crystals with fewer defects, and can improve the adhesion of thecrystalline island to the substrate.

The island material can include, but is not limited to, semiconductors.Such semiconductor can include, but are not limited to, silicon. Thesubstrate material can include, but is not limited to, silica, alumina,sapphire, niobium, molybdenum, tantalum, tungsten, rhenium, titanium,vanadium, chromium, zirconium, hafnium, ruthenium, osmium, iridium, andcombinations and alloys of these materials.

The substrate can also be a ceramic or glasses with sufficiently highmelting or softening temperatures. The substrate can also be aHigh-Temperature Co-fired Ceramic (HTCC). HTCC can be worked andmechanically patterned in its green phase. When the island material issilicon, alumina can be a relatively higher wetting angle material andsilica a relatively lower wetting angle material.

The above-described embodiments of the invention are intended to beexamples of the present invention and alterations and modifications maybe effected thereto, by those of skill in the art, without departingfrom the scope of the invention which is defined solely by the claimsappended hereto.

I claim:
 1. A method of fabricating a crystalline island of an islandmaterial, the method comprising depositing particles of the islandmaterial abutting a substrate; heating the substrate and the particlesof the island material to melt and fuse the particles to form a moltenglobule; cooling the substrate and the molten globule to crystallize themolten globule, thereby securing the crystalline island of the islandmaterial to the substrate; and planarizing at least a portion of thecrystalline island to expose a cross-section of the crystalline island.2. The method of claim 1, wherein the crystalline island comprises asingle crystal of the island material or a polycrystalline form of theisland material.
 3. The method of claim 1, wherein the securingcomprises the molten globule wetting the substrate at a wetting anglesmaller than about 90 degrees and the crystalline island adhering to thesubstrate.
 4. The method of claim 1, wherein the depositing comprises:defining a depression in the substrate; and transferring the particlesof the island material into the depression.
 5. The method of claim 4,wherein the securing comprises a portion of a surface of the depressionenveloping a portion of a surface of the crystalline island.
 6. Themethod of claim 4, wherein the depression is shaped to have at least onevertex.
 7. The method of claim 4, wherein the depression comprises: afirst depression; and a second depression within the first depression,the second depression smaller and deeper than the first depression. 8.The method of claim 5, wherein the transferring comprises one or moreof: doctor-blading the particles of the island material into thedepression; and electrostatic deposition of the particles of the islandmaterial into the depression using a charged pin.
 9. The method of claim4, wherein the transferring comprises flowing a suspension onto thesubstrate and into the depression, the suspension comprising adispersion of the particles of the island material in a carrier medium;and squeegeeing the suspension located on the substrate outside thedepression; and the heating further comprises: eliminating the carriermedium prior to the melting and fusing the particles of the islandmaterial.
 10. The method of claim 1, wherein the cooling comprises oneor more of oxidizing an outer surface of the molten globule;super-cooling the molten globule; and applying a physical impact to thesubstrate.
 11. The method of claim 1, wherein: the depositing comprisestransferring particles of the island material into a through hole in thesubstrate.
 12. The method of claim 11, wherein pressure is applied at asecond end of the through hole to push the molten globule partially outof a first end of the through hole to form a convex meniscus.
 13. Themethod of claim 11, wherein the securing comprises a portion of asurface of the through hole enveloping a portion of a surface of thecrystalline island.
 14. The method of claim 11, further comprising:after the cooling to crystallize the molten globule, planarizing aportion of the meniscus to expose a cross-section of the crystallineisland.
 15. The method of claim 11, wherein a portion of a surface ofthe substrate outside the through hole and adjacent a first end of thethrough hole has a wetting angle with the molten globule of less thanabout 90 degrees.
 16. The method of claim 1, wherein: the depositingcomprises: dispersing the particles of the island material in a carriermedium to create a suspension; and transferring the suspension onto thesubstrate; and the heating further comprises: eliminating the carriermedium prior to the melting and fusing the particles of the islandmaterial.
 17. The method of claim 16, wherein the transferring comprisesone or more of stamping the suspension onto the substrate; screenprinting the suspension onto the substrate; inkjet printing thesuspension onto the substrate; and spin-coating the suspension onto thesubstrate, and lithographically patterning the spin-coated suspension.18. The method of claim 1, wherein: the depositing comprises: dispersingthe particles of the island material in a carrier medium to create asuspension; forming the suspension into a sheet; causing the sheet tosolidify to form a solid sheet; patterning the solid sheet by removingone or more portions of the sheet to form a patterned sheet; andoverlaying the patterned sheet on the substrate; and the heating furthercomprises: eliminating the carrier medium prior to the melting andfusing the particles of the island material.
 19. The method of claim 1,wherein: the molten globule has a first wetting angle with a firstportion of a surface of the substrate in contact with the moltenglobule, and a second wetting angle with a second portion of the surfaceof the substrate, the second portion abutting the first portion, and thesecond wetting angle being greater than the first wetting angle.
 20. Themethod of claim 1, wherein an area of the substrate in contact with themolten globule comprises one or more of: one or more guidingprotrusions, one or more guiding depressions, and a metallic grid, forcontrolling initiation of crystallization as the molten globule iscooled.
 21. The method of claim 1, wherein the coefficient of thermalexpansion (CTE) of the substrate at a temperature within about 20°C. ofthe melting point of the island material matches the CTE of the islandmaterial at the melting point of the island material.
 22. The method ofclaim 1, wherein the island material comprises silicon.
 23. The methodof claim 1, wherein the substrate comprises alumina.
 24. The method ofclaim 1, where the securing comprises over-coating the crystallineisland and the substrate with an over-coating layer to form a stackwhereby the crystalline island is sandwiched between the substrate andthe over-coating layer.
 25. The method of claim 24, further comprisingplanarizing the stack to expose a cross-section of the crystallineisland.